1. Field of the Invention
The present invention relates to a transmission power control method for conducting transmission power control of a downlink while a mobile station is effecting a diversity handover, and a mobile station suitable for use with this method.
2. Description of the Related Art
FIG. 1 shows a configuration of a mobile communication system such as a portable telephone system currently in wide use. In the mobile communication system shown in FIG. 1, the whole service area is divided into comparatively small radio zones called “cells 1 to 5”. Such a mobile communication system includes a plurality of base stations 101 to 105 respectively covering the cells 1 to 5, and mobile stations 301 to 305 which set radio channels and communicate with the base stations 101 to 105 respectively.
In such a mobile communication system, radio waves transmitted from the base stations 101 to 105 are attenuated when they are propagated through space, and they arrive at the mobile stations 301 to 305. The radio waves are influenced in the degree of attenuation not only by the distances between the base stations 101 to 105 and the mobile stations 301 to 305, but also by the configuration of the land and buildings around the base stations 101 to 105 and the mobile stations 301 to 305.
When the “transmission power of radio waves” (hereafter referred to as transmission power) from the base stations 101 to 105 is constant, the “reception power of radio waves” (hereafter referred to as reception power) of the mobile stations 301 to 305 varies violently according to the movement of the mobile stations 301 to 305. Such variation is called “fading”.
Conventionally, as a technique for keeping the communication quality constant even in an environment with fading, there is known a transmission power control method of a feedback type (conventional first transmission power control method) based upon the “reception quality of radio waves” (hereafter referred to as reception quality).
To be more concrete, so as to track the variation of the propagation level caused by fading or the like, in the conventional first transmission power control method, the reception side (such as a mobile station) measures the reception quality, compares the measured reception quality with a desired value, and feeds back the comparison result to the transmission side (such as a base station) with a sufficiently short period of radio frames, time slots, or the like, and the transmission side adjusts the transmission power on the basis of the comparison result.
The first transmission power control method not only mitigates the influence of fading and keeps the reception quality constant, but also is effective in mitigating the variation in the reception quality caused by the location of the mobile station 301 to 305 in the service area, suppressing the transmission power to the minimum, and improving power utilization efficiency.
As a reference for the reception quality, a “signal to interference power ratio” (SIR), “reception power”, and a “result of error detection using CRC” (cyclic redundancy check) can be utilized.
Typically, in the mobile communication system, each of the mobile stations 301 to 305 suitably switches base stations 101 to 105 to which a radio channel is established, as it moves. This operation is called “handover”.
As the “handover”, the “hard handover (HHO) scheme” and the “diversity handover (DHO) scheme” have been considered. In the hard handover (HHO) scheme, a mobile station 30 moving across a boundary between the cells 1 to 5 instantaneously switches the base stations 101 to 105 to which a radio channel is established, and a radio channel is constantly established between the mobile station 30 and a single base station 10. In the diversity handover (DHO) scheme, a mobile station 30 moving across a boundary between the cells 1 to 5 establishes a radio channel between a new base station 102 and the mobile station 30 before opening a radio channel between a base station 101 under communication and the mobile station 30 and thus the mobile station 30 temporarily communicates with a plurality of base stations 101 and 102 at the same time.
The DHO scheme has an advantage over the HHO scheme that interruption is not caused at the time of switching of the base stations 101 to 105.
If the mobile station 30 is located at an end of the cells 1 to 5 and the mobile station 30 communicates with a single base station 10, then the base station 10 needs large transmission power in order to keep the reception quality constant, which causes problems.
In such a case, there is a possibility that sufficient reception power capable of coping with a fall in the propagation level caused by fading cannot be obtained in the mobile station 30.
If the DHO scheme is applied, however, then the mobile station 30 can simultaneously receive radio waves (signals) transmitted from a plurality of base stations 10 and combine them. As a result, the problem can be solved.
Fading differs for each base station 10. By using the DHO scheme, therefore, falls in the propagation level caused by fading can be compensated for between a plurality of base stations 10. Thus there can be obtained effects such as the stabilization of communication quality and reduction of transmission power to the base station 10.
For the transmission scheme in the mobile communication system, there is a “dedicated scheme” and a “shared scheme”. In the “dedicated scheme”, a dedicated channel (DCH) is established for each mobile station 30. In the “shared scheme”, one (or more) shared channels (SCH) having a large transmission capacity is prepared and a plurality of mobile stations 30 share the “SCH” in a time division form using scheduling.
The “dedicated scheme” has an advantage in that the transmission rate for each mobile station 30 is ensured. However, the “dedicated scheme” has a drawback in that the transmission rate for each mobile station 30 is kept down to a low value and as many hardware resources (radio channels) as the number of the mobile stations 30 that can communicate simultaneously are needed.
On the other hand, the “shared scheme” has a drawback in that the transmission rate for each mobile station 30 is not ensured. However, the “shared scheme” has an advantage in that a high transmission rate for each mobile station 30 can be achieved when the number of the mobile stations 30 that communicate simultaneously is small, and the required hardware resource (radio channel) is only one “SCH”.
The “dedicated scheme” is suitable for communication that varies slightly in transmitted information content with time, makes strong demands regarding transmission delay, and always needs a constant communication band, such as audio communication.
On the other hand, the “shared scheme” is suitable for intermittent communication whose transmitted information content varies greatly with time and comparatively does not make strong demands regarding transmission delay.
If information directed to a specific mobile station 30 exists on the SCH in the shared scheme, then the mobile station 30 is “notified” (signaled) to that effect. The signaling may be conducted on a dedicated DCH established for each mobile station 30, or may be conducted on an established SCH for signaling.
Information transmitted on the SCH for a specific mobile station 30 might become intermittent, because a plurality of mobile stations 30 share the SCH. If the conventional first transmission power control method is applied using the reception quality of the SCH when controlling the transmission power of the SCH, then the transmission power control becomes intermittent and trouble is caused, resulting in a problem.
In order to solve this problem, the “second transmission power control method” can be used. During an interval having a possibility that an SCH will be transmitted to a mobile station 30, a DCH is established incidentally for the mobile station 30 and the conventional first transmission power control method is applied continuously by using the reception quality of the DCH. If there is transmission of an SCH, then the transmission power of the SCH is linked with the transmission power of the DCH with a certain offset.
According to the second transmission power control method, the transmission power of the SCH can be controlled indirectly by linking the transmission power of the SCH directed to the mobile station 30 with the transmission power of the DCH directed to the mobile station 30 as shown in FIG. 2.
In FIG. 2, the transmission power (FIG. 2B) of a “DCH (physical channel A)” has a shape obtained by nearly inverting vertically that of a variation of a propagation level (FIG. 2A) caused by fading or the like. As a result, the “DCH (physical channel A)” has a constant reception quality FIG. 2C.
In other words, in FIG. 2, the variation (FIG. 2A) of the propagation level of the “SCH (physical channel B)” is similar to the “variation of the propagation level of the “DCH (physical channel A)”. If the transmission power (FIG. 2B) of the “SCH (physical channel B)” is linked with the transmission power (FIG. 2B) of the “DCH (physical channel A)”, then the reception quality (FIG. 2C) of the “SCH (physical channel B)” also becomes constant.
Such a conventional second transmission power control method can also cope with a multi-call in which the same mobile station 30 conducts a plurality of communication operations, such as the case where the mobile station 30 receives electronic mail while the mobile station 30 is conducting audio communication on the DCH.
When application of the DHO scheme is considered, it becomes necessary in the shared scheme to adjust scheduling of transmission timing between the base stations 101 to 105 and consequently the control load of the network increases.
Furthermore, in a mobile communication system in which the number of base stations 101 to 105 is large and the cells 1 to 5 are continuous, it is difficult for the mobile station 30 to adjust the scheduling of the timing of transmission to the same mobile station 30 between a plurality of base stations 101 to 105.
Typically in the shared system, therefore, it is simpler to apply the HHO scheme.
However, it is effective to apply the DHO scheme to the DCH in expectation of no interruption occurring at the time of handover, the stabilization of quality of the DCH, and the reduction of the transmission power required.
In such a case, one of the base stations 10 transmitting the DHOs may transmit an SCH to a specific mobile station 30 that is conducting DHO using a plurality of DCHs.
When the mobile station 30 is conducting DHO using specific physical channels A, i.e., first signals (DCHs in the above described example), there is a method of simultaneously communicating by using a different physical channel B, i.e., a second signal (SCH in the above described example).
As in the above described example of DCH and SCH, a base station group B (such as 301) transmitting the physical channel B is a subset of a base station group A (such as 301 to 305) transmitting the physical channels A. However, the base station group A does not coincide with the base station group B in some cases.
According to the conventional second transmission power control method, transmission power of the physical channel A and transmission power of the physical channel B are simultaneously controlled in such a case, on the basis of a result of a reception quality measurement of the physical channel A obtained after a diversity combination.
In other words, in the conventional second transmission power control method, the transmission power of the physical channel A is controlled on the basis of the result of the reception quality measurement of the physical channel A obtained after the diversity combination, and the transmission power of the physical channel B is controlled indirectly by linking with the transmission power of the physical channel A.
In the conventional second transmission power control method, the reception quality of the physical channel A obtained after the diversity combination is kept constant. However, the reception quality of the physical channel B cannot be kept constant. An example is shown in FIG. 3.
FIG. 3 shows an example of the case where the base station 101 transmits the physical channel B (SCH) to the mobile station 302, which is conducting DHO between two base stations, i.e., the base station 101 and the base station 102, using the physical channels A (DCHs) (see FIG. 1).
The propagation level of the base station 101 (FIG. 3A) and the propagation level of the base station 102 (FIG. 3B) are varied by independent fading phenomena, respectively.
The mobile station 302 conducts a diversity combination on received signals of the “physical channels A (DCHs)” transmitted from the base stations 101 and 102, and controls the transmission power of the “physical channels A (DCHs)” of the base stations 101 and 102 so as to keep the received signal quality obtained after the diversity combination constant (see FIG. 3E).
The “physical channel B (SCH)” is transmitted only from the base station 101. The transmission power of the “physical channel B (SCH)” is controlled so as to be linked with the transmission power of the “physical channel A (DCH)” of the base station 101 controlled as described above (see FIG. 3C and FIG. 3D).
Therefore, the reception quality of the “physical channel B (SCH)” received in the mobile station 302 does not become constant, but varies violently.
In other words, the case where the reception quality of the “physical channel B (SCH)” does not satisfy the “required quality of the physical channel B” frequently occurs, and the communication quality of the “physical channel B (SCH)” is degraded (see FIG. 3E).
Furthermore, there frequently occurs the case where although the reception quality of the “physical channel B (SCH)” satisfies the “required quality of the physical channel B”, the reception quality of the “physical channel B (SCH)” is unnecessarily high and the “physical channel B (SCH)” is transmitted with excessive power (i.e., “excessive quality” state). Excessive transmission power lowers the power utilization efficiency in the mobile communication system, and in addition increases interference on the surroundings. Therefore, excessive transmission power lowers the efficiency of the whole mobile communication system.
Thus, the conventional second transmission power control method has a fatal problem in that the reception quality of the “physical channel B (SCH)” is degraded when the mobile station 30 is conducting DHO using the “physical channels A (DCHs)”.
Furthermore, there is a problem that the transmission power of the “physical channel B (SCH)” becomes excessive and power utilization efficiency in the mobile communication system is lowered. Furthermore, this results in a problem that excessive transmission power increases interference and lowers the efficiency of the whole mobile communication system.